Alkanolamine process units remove H.sub.2 S and CO.sub.2 from gaseous process streams, typically by countercurrently contacting an aqueous solution containing from about 20% to about 50% by weight of an alkanolamine with a gas stream containing H.sub.2 S and/or CO.sub.2.
Various amines, such as monoethanol amine (MEA), diethanol amine (DEA), and triethanolamine (TEA), merely to name a few, are useful in acid gas sorption. While each of these amines is an effective acid gas sorbent, sorption process conditions typically require the use of one or two selected alkanolamines due to the different boiling points of the various alkanolamines.
The removal of hydrogen sulfide from gaseous streams, such as the waste gases liberated in the course of various chemical and industrial processes, for example, in wood pulping, natural gas and crude oil production and in petroleum refining, has become increasingly important in combating atmospheric pollution. Hydrogen sulfide containing gases not only have an offensive odor, but such gases may cause damage to vegetation, painted surfaces and wildlife, and further may constitute a significant health hazard to humans. Government-wide regulations have increasingly imposed lower tolerances on the content of hydrogen sulfide which can be vented to the atmosphere, and it is now imperative in many localities to remove virtually all the hydrogen sulfide under the penalty of an absolute ban on continuing operation of a plant or the like which produces the hydrogen sulfide-containing gaseous stream. Solutions of water and one or more the alkanolamines are widely used in industry to remove hydrogen sulfide and carbon dioxide from such gaseous streams.
Corrosion in alkanolamine units significantly increases both operating and maintenance costs. The mechanisms of corrosive attack include general corrosive thinning, pitting corrosion-erosion, and stress-corrosion cracking. Corrosion control techniques include the use of more expensive corrosion and erosion resistant alloys, continuous or periodic removal of corrosion-promoting agents in suspended solids by filtration, activated carbon adsorption, or by the addition of corrosion inhibitors. (See Kohl, A. L. and Reisenfeld, F. C., Gas Purification, Gulf Publishing Company, Houston, 1979, pp. 91-105, as well as K. F. Butwell, D. J. Kubec and P. W. Sigmund, "Alkanolamine Treating", Hydrocarbon Processing, March, 1982.)
Further, it has been found that the acid gas sorption capacity in a circulating alkanolamine-water system decreases with time on stream in the absence of added makeup alkanolamine. This performance degradation has been found to be attributable to the accumulation of heat stable salts and complex amine degradation products. U.S. Pat. No. 4,795,565 to Yan describes a process for removing heat stable salts from an ethanolamine system by the use of ion exchange resins. The disclosure of U.S. Pat. No. 4,795,565 to Yan is incorporated herein by reference for the operating details both of an ethanolamine acid gas sorption system as well as for the heat stable salt removal process.
Heat stable salts may also be removed from certain aqueous alkanolamine systems by distillation. However, such separation has been limited in the past to relatively mild conditions of temperature and pressure to avoid thermal degradation of the alkanolamine. For example, while distillation effectively purifies monoethanol amine (MEA), fractionation of the higher boiling alkanolamines is complicated by their tendency to thermally degrade at elevated temperature. Diethanolamine (DEA), for example, boils at 268.degree. C. at 760 mm Hg pressure and tends to oxidize and decompose at high temperature.
U.S. Pat. No. 4,079,117 to Butwell teaches a continuous process for removing acid gases, for example, carbon dioxide, from a hydrocarbon gas containing the same. Stripping and absorption temperatures in the Butwell process are maintained at below about 150.degree. C.
U.S. Pat. No. 4,133,837 to Kendall et al. teaches a process for removing polymer from aqueous alkanolamine solutions which process includes the steps of adjusting the solution pH and removing the polymer by decantation and/or filtration.
U.S. Pat. No. 4,343,777 to Dannhorn et al. also relates to a process for removing accumulated polymer from an aqueous alkanolamine solution used for sorbing acid gases such as CO.sub.2 and H.sub.2 S. After the spent alkanolamine solution is stripped of acid gas, the alkanolamine solution is contacted with a water immiscible organic solvent to extract the accumulated polymeric materials from the solution.
The chemistry of alkanolamine degradation is discussed in the Butwell et al. article cited above. Briefly, the Butwell et al. article notes that monoethanolamine (MEA) irreversibly degrades to N-(2-hydroxyethyl) ethylene diamine (HEED). HEED shows reduced acid gas removal properties and becomes corrosive at concentrations of at least about 0.4% by weight.
Diglycolamine (DGA), on the other hand, is said to produce a degradation product upon reaction with CO.sub.2 which exhibits different properties. DGA is a registered trademark of Texaco, Inc. which identifies an amine having the chemical formula NH.sub.2 --C.sub.2 H.sub.4 --O--C.sub.2 H.sub.4 --OH. DGA degrades in the presence of CO.sub.2 to form N,N'-bis(hydroxyethoxyethyl) urea (BHEEU) which is similar to HEED in corrosivity but differs in that BHEEU has no acid gas removal properties.
Diethanolamine (DEA) reacts with CO.sub.2 to form N,N'-di(2-hydroxyethyl) piperazine. Unlike HEED and BHEEU, the piperazine compound is noncorrosive and has acid gas removal properties essentially equal to its parent, DEA. See the Butwell et al. article at page 113.
Diisopropylamine (DIPA) readily degrades in the contact with CO.sub.2 to form 3-(2-hydroxypropyl) 5-methyl oxazolidone which shows essentially no acid gas removal properties. See the Butwell et al. article at page 113.
Numerous degradation products formed by the reaction of H.sub.2 S, or a mixture of H.sub.2 S and CO.sub.2 with diethanolamine have been reported from analyses of operating diethanolamine acid gas sorption processes and are shown below in Table 1.
TABLE 1 __________________________________________________________________________ COMPOUNDS RESULTING FROM DEA DEGRADATION Name Abbreviation Structural formula __________________________________________________________________________ N,N-Bis (2-hydroxy- ethyl) piperazine HEP ##STR1## N,N,N-tris (2-hydroxy- ethyl) ethylenediamine THEED ##STR2## Hydroxyethyl imida- zolidone HEI ##STR3## N-Methyldiethanolamine MDEA ##STR4## Oxazolidone OZO ##STR5## Aminoethykethanolamine AEEA ##STR6## Bis-(2-hydroxy ethyl) glycine BHG ##STR7## __________________________________________________________________________
Accumulation of these and other degradation products in the alkanolamine system reduces acid gas sorption capacity and increases the corrosivity of the alkanolamine solution. Previous processes have addressed removal of heat stable salts and amine degradation product, but such removal necessarily generates a waste stream, and the degraded alkanolamine withdrawn from the system must be replaced with fresh makeup alkanolamine. Thus is would be desirable to provide a method for restoring acid gas sorption capacity to a spent alkanolamine solution while minimizing the quantity of waste material withdrawn from the process. Further, it would be beneficial if a process for restoring alkanolamine acid gas sorption capacity would promote rejection of heat stable salts from the alkanolamine solution.